Epidermal growth factor (EGF) plays an important role in the proliferation and differentiation of many cell types. The goal of this proposal is to use directed evolution to generate EGF mutants with enhanced binding affinities to the extracellular domains of the epidermal growth factor receptor (EGFR). The rationale is that EGF mutants with increased binding affinity to different receptor subdomains will elicit distinct cellular responses. Soluble extracellular EGFR domain fragments will be generated and used to screen and sort EGF mutant libraries produced by yeast surface display technologies. EGF mutants that specifically bind to the EGFR fragments with high affinity will be produced in soluble form, and tested for their ability to modulate biological functions. The EGFR is over-expressed and/or constitutively activated on many. tumors, including brain, mammary and ovarian. The generation of antagonist EGF ligands that bind to the receptor without inducing activation could have potent cancer therapeutic applications. Additionally, EGF has been implicated in wound healing through promotion of fibroblast proliferation and migration to the affected area. Superagonist EGF mutants that elicit enhanced efficacy at lower doses could be useful in tissue engineering applications.